It is conventional for shaft speed measurement purposes to use a transducer device which produces electrical signals at specified angular positions of the shaft. These signals have been used in two ways to obtain digital speed information. In one known procedure the number of signals occurring in a fixed time is counted and in the other known procedure the time between two successive signals is measured by counting the number of pulses produced in that period by a constant frequency clock oscillator.
Both known methods give rise to problems of accuracy of resolution and frequency of sampling. If, for example, it were required to determine the speed within a period of 2.5 mS to an accuracy of 3 rpm when the maximum shaft speed was 3000 rpm the first known method would require a transducer device which emitted 8000 signals per revolution, and the second known method would require a clock frequency 4 MHz (assuming a speed range of 10:1). In the second method there would also be the problem that at the low speed end of the range the "period" count would be very large and at the high speed end the sampling frequency would become unnecessarily high.
It is thus an object of the invention to provide a transducer circuit capable of measuring rotary speed to obtain a desired resolution and sampling period without involving unnecessarily high signals per revolution from the transducer used or unnecessarily high clock frequencies.